The present invention relates to an improved apparatus for detecting a relative rotational position between two shafts, which is suitable for use as, for example, a torque sensor for detecting a torsional force or load applied to a power steering shaft of a motor vehicle.
Of various types of techniques for detecting torsional amounts of two relatively rotatable shafts, there have been well known those which are characterized by provision of detection devices, such as a potentiometer or resolver devices, on input and output shafts interconnected via a torsion bar. According to the above-mentioned technique using a potentiometer, a slider is mounted on the input shaft while a resistor is mounted on the output shaft, so that a position of the slider contacting the resistor varies in accordance with a variation in a relative rotational position between the input and output shafts to thereby provide an analog voltage corresponding to the relative rotational position. According to the technique using resolver devices, separate resolver devices are provided on both of the input and output shafts so as to detect a relative rotational amount (torsional amount) between the two shafts on the basis of angle signals produced by the two resolver devices. Further, as a means for detecting a relative rotational displacement between two relatively rotatable shafts, there has been developed a noncontact-type torque sensor for electronic power steering which employs an induction coil.
The conventional technique of the type using a potentiometer would always suffer from poor electrical contact, failure and/or other problem since the electrical contact is implemented via a mechanical contact structure. Further, because there occurs impedance variations due to temperature changes, it is necessary to appropriately compensate for a temperature drift. Further, the rotational-displacement detection apparatus, known as the noncontact-type torque sensor for electric power steering employing the induction coil, is arranged to measure an analog voltage level produced in response to a minute relative rotational displacement, so that it only accomplishes a very poor detecting resolution. Further, in addition to the need to compensate temperature drift characteristics of the coil, there is a need to appropriately compensate temperature drift characteristics present in reluctance of magnetic substances that vary magnetic coupling to the coil in response to a changing relative rotational position as well as in eddy current loss of electrically conductive substances. Furthermore, it is desirable that the torque sensors for motor vehicles be arranged as a dual-sensing structure for safety purposes.
It is therefore an object of the present invention to provide a relative-rotational-position detection apparatus which can achieve superior temperature-characteristic compensating performance and can be easily arranged as a dual-sensing structure.
It is another object of the present invention to provide a relative-rotational-position detection apparatus which is capable of high-resolution detection even when a relative rotational displacement to be detected is very minute.
According to an aspect of the present invention, there is provided a relative-rotational-position detection apparatus for detecting a relative rotational position between a first shaft and a second shaft rotatable relative to each other, which comprises: an outer cylindrical section rotatable with the first shaft, the outer cylindrical having a nonmagnetic and nonconductive cylindrical base and magnetic shielding portions formed of a magnetic shielding or antimagnetic substance and arranged on a surface of the cylindrical base, the magnetic shielding portions being spaced apart from each other by a predetermined interval in a circumferential direction of the cylindrical base so that non-magnetically-shielding portions are formed between the magnetic shielding portions; a plurality of coils provided on a periphery of the outer cylindrical section and excitable by a predetermined A.C. signal; and an inner cylindrical section inserted in the outer cylindrical section and rotatable with the second shaft, the inner cylindrical section including magnetic portions each provided to present a different characteristic with respect to an arrangement of the plurality of coils. Here, in response to a variation in a relative rotational position between the first shaft and the second shaft, a degree of overlap between the non-magnetically-shielding portions of the outer cylindrical section and the magnetic portions of the inner cylindrical section varies, and each of the coils presents impedance corresponding to the degree of overlap.
The relative-rotational-position detection apparatus of the present invention is extremely useful in that it permits accurate detection by appropriately compensating temperature drift characteristics and in that it can be constructed to provide dual detection outputs.
While the described embodiments represent the preferred form of the present invention, it is to be understood that various modifications will occur to those skilled in the art without departing from the spirit of the invention. The scope of the present invention is therefore to be determined solely by the appended claims.